The present invention relates generally to intercept receivers, and more particularly, to frequency agile emitters used in radar systems.
Electronic Surveillance Measures (ESM) intercept receivers are used to collect radar data, and in particular to perform a pulse parameter measurement function. Pulse parameter measurements are used to type or xe2x80x9cfingerprintxe2x80x9d radar systems. The measurements taken by the ESM receiver include the traditional parameters such as pulse time of arrival (TOA), pulse width (PW), signal amplitude and carrier frequency, and so-called pulse internals such as modulation. Modulation measurements involve, for frequency agile radars, finding the minimum and maximum frequencies, time between the minimum and maximum frequencies, and the number of frequency steps for both LFM (linear frequency modulation, or chirp) and FSK (frequency-shift-keyed) signals. All these, and other electronic intelligence (ELINT) measurements are most accurate when performed on many pulses and statistically combined. Therefore the controller for the ESM receiver operated in an ELINT mode may direct a continuous tune to collect data for a single radar over several minutes.
The controller directs the receiver to tune to a certain center frequency. This is called a xe2x80x9cdwellxe2x80x9d. During a dwell, all pulses with RF carrier frequencies falling in the receiver bandwidth centered at the tune frequency are collected, sorted and processed. When fingerprinting, the sorting must occur in microseconds to assure enough pulses are stored to allow processing of sufficient contiguous pulses from the same radar. Such screening can rely on only one or two parameters. For fixed frequency radars the sorting parameter is typically RF carrier frequency. But frequency as a sorting parameter is not useful for frequency agile radars.
Radars use frequency agility either as an electronic counter countermeasure (ECCM), or to enhance performance. As an example of performance use, many sea borne radars have a frequency change every 10 ms to 100 ms to electronically steer the antenna beam. An example of ECCM use is frequency hopping within a bandwidth, possibly extending over 1 GHz, to reduce the vulnerability of surface-to-air missile systems to jamming.
As these two examples indicate, the change in transmitted frequency can be on a pulse-by-pulse or pulse-batch to pulse-batch basis, with the RF carrier frequency of the pulse perturbed in either a random or preprogrammed fashion. But even if deterministic at the transmitter, the frequency change schedule is typically such that the frequency of the next pulse or pulse group cannot be reliably predicted from the frequency of the current pulse by ESM processing.
Therefore in current systems the ESM receiver control must direct a wideband tune to capture pulses contiguous in time from frequency agile radar. But then the number of pulses for a particular agile radar that can be sampled and stored is limited since so many pulses from other radars in the environment will be collected. In fact, with a wideband tune the number of pulses stored from a single agile emitter is generally too small to do precision parameter extraction.
The linking of frequency agile pluses is called dehopping. The traditional way to use interferometer measured phase for dehopping is with a multichannel system generating the emitter angle-of-arrival or aoa from a single pulse. But implementing this is cumbersome since typically at least five channels are needed to generate sufficiently accurate aoa for sorting; i.e. five separate receivers are required to measure monopulse phase across four different interferometer baselines. Since, in an effort to limit both weight and cost, many recent ESM systems now incorporate only two channel receivers, such as Litton Industries Advanced Systems Division""s LR-100 ESM Receiver, this conventional interferometer monopulse approach to dehopping is typically not available, and an alternative must be found.
A commonly tried alternative to using multichannel generated aoa for monopulse sorting is the use of pulse repetition interval (PRI) tracking and prediction. But radars use a variety of ECCM techniques, especially against jammers, that have the secondary consequence of rendering PRI tracking useless. For example, some radar time jitter their pulses. And some radar, in particular, many airborne radars, may operate in an agile-agile mode. That is, the radar can vary both the carrier frequency and the pulse repetition frequency. Therefore, sorting by PRI, i.e. predicting a time window for the next pulse based on a stable pulse-time interval model (using, for example, some variant of the approach described by Udd et al in U.S. Pat. No. 5,091,917 xe2x80x9cMethod and Apparatus for Pulse Sortingxe2x80x9d), is, for a large and important class of emitters, not viable either. Hence, with neither frequency or pulse repetition interval prediction available, real-time agile-agile radar pulse sorting has not been possible in two-channel ESM systems.
Receivers like the LR-100 are particularly well adapted to use with an LBI, since LBI installations only require two antennas, and hence two receiver channels. The measured phase is highly ambiguous since the LBI baseline can be hundreds or even thousands of RF carrier wavelengths long, i.e., integer n 104, FIG. 1, is unknown and possibly very large. This integer n 104 is unknown because the receiver can only measure phase within a cycle (equivalently 2xcfx80 radians or 360xc2x0), but many cycles nm of phase are typically associated with the signal spatial angle-of-arrival 136. The number of cycles is a function of the antenna separation, or baseline length d 137, and is proportional to the number of wavelengths at the signal frequency 102 in d. In contrast to the single LBI baseline hundreds of wavelengths long, a conventional short-baseline-interferometer, or SBI, has several baselines, each with antennas spaced at most tens of wavelengths apart. The SBI uses comparisons between phase measurements made across this multiple set of short baselines during a single dwell to resolve the phase cycle ambiguities. But the long LBI baseline cannot be conveniently phase-calibrated like the SBI; also the antennas forming the LBI are so widely separated that they usually cannot be phase-error-nulled, or clocked and boresighted. Not calibrating the antenna-to-receiver cables or clocking and boresighting the antennas introduces a potentially large bias error into the phase measurements. Thus LBI based applications must use phase measurement differences rather than single phase measurements like the SBI, so that these bias errors cancel in the differenced measurements. In previous applications, e.g. Kaplan U.S. Pat. No. 4,734,702 and the applicant""s U.S. Pat. No. 5,343,212 discussed below, the differencing occurred across receiver dwells.
The use of this dwell-difference approach in past LBI applications has generally involved passive emitter location. When the LBI is used for passive location, if there is no significant aircraft attitude change between dwells all bias errors are essentially dwell-to-dwell constant and hence cancel almost totally for fixed-frequency emitters. When the LBI phase difference measurements have been used to locate emitters in range, as described in Kaplan, U.S. Pat. No. 4,734,702, and Rose et al in U.S. Pat. No. 5,343,212 the ambiguity resolution was accomplished as part of the location process (Rose) or by predicting the cycle integer using SBI outputs (Kaplan). This invention extends the use of LBI phase differences to monopulse sorting of both moving and fixed emitters. But, as noted, unlike previous applications of the LBI the invention does not involve locating the emitter in range. Since the invention does not involve emitter location in range, and explicitly avoids the need for an SBI, it requires a different approach from that of Kaplan or Rose to resolve the phase difference ambiguity.
Note that that this ambiguity resolution is required, i.e. the LBI ambiguous differential phase 112 (FIG. 1) by itself is not an agile emitter sorting parameter. The differential ambiguous phase does not provide pulse-to-emitter linking if the frequency associated with the sequential phase measurements varies, because this phase involves the product of two parameters: frequency and cos(aoa). When the cycle uncertainty 104 is added to this, in a dense emitter environment there will be many incorrect associations since many emitters at diverse frequencies and relative bearings will map, within system errors, into the same ambiguous phase difference.
Thus, it is an object of the present invention to monopulse dehop frequency agile radar pulses using both estimates and predictions of the cycle integer resolving two channel ambiguous differential phase measurements made on an LBI baseline by synthesizing an equivalent short baseline interferometer from the LBI.
Hence it is a further object of the present invention to extend the technique of forming the equivalent array from a conventional SBI to generating a synthesized short baseline interferometer (SSBI) from the LBI.
In the invention the LBI phase difference ambiguity integer 138 is used as the main discriminate. Computing the integer effectively separates the frequency-cos(aoa) product, and also separates this product from the cycle uncertainty. The way this sorting parameter is found, independent of SBI measurements (Kaplan), or a location hypothesis test (Rose), and the method by which it eliminates the deficiencies in processing agile and agile-agile signals, and further allows the ultimate use of LBI phase difference measurements 112 as an additional screening gate.
The present invention extends the method disclosed in the applicant""s patent application xe2x80x9cMethod of Detection and Determining Angular Location of Frequency Agile Emittersxe2x80x9d, Ser. No. 09/487,209, filed Jan. 19, 2000, which is hereby incorporated by reference in its entirety into the present specification. The invention disclosed in Ser. No. 09/487,209, provides real-time (rather than off-line) monopulse frequency agile pulse sorting that guarantees contiguous pulses from the agile radar will be captured, while drastically limiting the inclusion of extraneous pulses from other radars in the same frequency band. The linking of frequency agile pulses is called dehopping. The present invention uses a phase measurement ambiguity integer (104, FIG. 1), and ambiguous pulse phase difference measurements 112 to dehop. The ambiguous phase measurements utilized in the aforementioned patent application entitled xe2x80x9cMethod of Detection and Determining Angular Location of Frequency Agile Emittersxe2x80x9d were made on a set of calibrated interferometer baselines in a single dwell. The present invention requires only a single interferometer baseline, and phase measurements on that interferometer baseline can have large unknown frequency-independent bias errors since differences 112 xcex94xcfx86, . . . xcex94xcfx86mxe2x88x921 between individual phase measurements are used.
As will be seen, the current invention overcomes the deficiency with two channel ESM systems that have frequency or pulse repetition prediction available. The present invention allows a single interferometer phase measurement made by receivers like the LR-100, to be used monopulse dehop, rather than requiring the simultaneous multichannel phase measurements previously needed. That is, the invention solves both the agile and agile-agile sorting problems with ambiguous monopulse phase measurements made on a single interferometer baseline. Both practically and theoretically, the best type of interferometer to use for making the phase measurement is an LBI, or long-baseline-interferometer. The LBI is the best practically because it is usually the least costly interferometer to implement. It is the best theoretically, because the invention utilizes the extensive LBI baseline length in implementing the dehop method.
In the current invention, which is a novel application of the LBI in the sense that it does not involve passive emitter ranging, the antenna-induced bias errors remain phase measurement update-to-update constant, but the antenna-to-receiver cable length differences produce a bias dependent on the signal frequency. This frequency-induced bias error does not cancel for agile emitters. It must be controlled to prevent gross errors when resolving the cycle ambiguities in the current invention.
Another object of the present invention is to use a phase measurement ambiguity integer and ambiguous pulse phase differences to dehop.
Yet another object of the present invention is to use only a single interferometer baseline which can include large unknown frequency-independent bias errors.
Still another object of the present invention is to use a single interferometer phase measurement rather than requiring simultaneous multi-channel phase measurement.
Yet a further object of the present invention is to solve both the agile and agile-agile sorting problems with ambiguous monopulse phase measurements made on a single interferometer baseline.
Yet still another object of the present invention is to use the accuracy of a resolved LBI aoa measurement in sensor coordinates to precisely predict an ambiguity integer associated with ambiguous phase measurement of the next pulse detected by a receiver at the measured frequency.
Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein the preferred embodiments of the invention are shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description thereof are to be regarded as illustrative in nature, and not as restrictive.